Age-chimeras are made by grafting mice with skin from syngeneic donors of ages different from that of the recipients. These grafted mice are used for a study of autonomous age-dependent alterations in skin susceptibility to the mitotic inhibitory effect of chalones fractionated on sephadex columns and for tests of the possible leukemogenic effect of skin first treated with DMBA and then transplanted. Fibroblasts aging in vitro are used for a study of the possible correlation between number of in vitro mitosis and chalone production/susceptibility and cell surface charge.